for Loop Example

The purpose of this exercise is to write a number of for loops to be able to understand how to implement them effectively.

1.  Create a script called defense.sh in /opt/scripts
The purpose of the script is to configure the firewall to drop known zombie networks.

#!/bin/bash
IP=/opt/scripts/banned

for i in $(awk '{print}' < "$IP" )
do
echo $i
iptables -A INPUT -p tcp -s $i -j DROP
done
exit 0

The script takes a list of IP Addresses in a file and uses awk to print each IP from the file into the variable $i.  Then an iptables command employs the “$i” variable to drop each of the IP Addresses on the INPUT chain.  The “-p tcp” limits the drop to TCP protocol and the “-s” indicates the source.  The IP is dropped with the jump “-j” to DROP.

Create a file with known zombie networks.  This file is actually maintained by Spamhaus.org (http://www.spamhaus.org/xbl/).  This is a short sample.

banned file with IPs
24.190.78.101
38.101.148.126
41.206.45.202
58.0.0.0/8
59.107.0.0/17
59.108.0.0/15
59.110.0.0/15
59.151.0.0/17
59.155.0.0/16
59.172.0.0/15

Test the script  output with :
iptables -L -n

Chain INPUT (policy ACCEPT)
target     prot opt source               destination
RH-Firewall-1-INPUT  all  –  0.0.0.0/0            0.0.0.0/0
DROP       tcp  –  24.190.78.101        0.0.0.0/0
DROP       tcp  –  38.101.148.126       0.0.0.0/0
DROP       tcp  –  41.206.45.202        0.0.0.0/0
DROP       tcp  –  58.0.0.0/8           0.0.0.0/0
DROP       tcp  –  59.107.0.0/17        0.0.0.0/0
DROP       tcp  –  59.108.0.0/15        0.0.0.0/0
DROP       tcp  –  59.110.0.0/15        0.0.0.0/0
DROP       tcp  –  59.151.0.0/17        0.0.0.0/0
DROP       tcp  –  59.155.0.0/16        0.0.0.0/0
DROP       tcp  –  59.172.0.0/15        0.0.0.0/0

2.  List executable files in a directory, create a file called dir.sh

#!/bin/bash
for i in *
do
if [ -f "$i" -a -x "$i" ]
then
echo "Executable file $i "
fi
done

This script will list executable files in a directory.  Note the for loop will loop through all files and only print those which are files “-f” and “-a” are executable “-x”.

Tagged as: for loop